Though Blair is unwilling to apologise for going to war in Iraq, he is happy to admit he got it wrong over Carole Caplin, the goat's-milk-drinking lifestyle guru whose association with a conman engulfed his family in a damaging personal crisis.

The couple's relationship with Caplin was not the mistake, however; Blair writes in his memoirs that she was "a good friend and reliable confidant" for his wife, and helped him stay in shape when "fitness became more of a preoccupation".

Where he erred was not openly acknowledging Caplin when her employment with the Blairs became public in 1994, just before the Labour party conference.

Cherie Blair had taken on Caplin to "remake her image: get fit, look good, carry herself like the well-known figure she was becoming".

The Sun got wind of it – and got hold of nude pictures of Caplin, which it published under the headline "Secrets of Blairs' Girl Friday".

Blair says that in retrospect he should have "acknowledged" her and been open and supportive. "Instead, entirely understandable given our nervousness about our position and how she was bound to provoke controversy, we hid her away in a safe house. But, of course, it only increased the fascination about her."

The interest never went away, and eight years later, Caplin's links to the family caused a deep crisis when it emerged that her boyfriend, Peter Foster, was a conman with a criminal conviction for, among other things, conspiring to supply a special weight-loss drink that turned out to be tea.

The problem was, Foster had helped Mrs Blair buy two flats in Bristol when the Blairs' son Euan was at university there. When the Daily Mail called Blair for a comment, he denied Foster had ever been involved.

This, writes Blair, was because the paper had phoned for a comment at the last possible moment the day before publication. Blair says he called his wife and, in a rush, got "the wrong end of the stick".

The denial fuelled the flames of what inevitably came to be known as Cheriegate. Looking back, his wife should "probably never have tried to buy the Bristol flats", says Blair, adding: "The trouble is you can't really do that as the prime minister's wife, for no better reason than you just can't."

In the four pages of the book which deal with Caplin, Blair also demolishes one of the more amusing rumours about his wife's relationship with her. The two women never showered together, says Blair, dismissing the idea as "fable".

He also describes Alastair Campbell's mistrust of Caplin. "My close office were, it is fair to say, intrigued but generally dismayed by Carole," he writes. "Alastair, in particular, couldn't understand her role and strongly disapproved of it. He judged, in a sense rightly, that politics had no place for someone as exotic and apolitical as Carole. Personally, that's why I found her so refreshing.

"Alastair was convinced she would sell her story. She never did. Whatever indignity was visited on her, she remained dignified. Contrary to the image assiduously and malignly created for her, she was kind, decent, hard-working and, above all, brilliant at what she did. The relationship with Foster was a big mistake, but it wasn't venal or badly motivated."